fftools/ffmpeg: add thread-aware transcode scheduling infrastructure

See the comment block at the top of fftools/ffmpeg_sched.h for more
details on what this scheduler is for.

This commit adds the scheduling code itself, along with minimal
integration with the rest of the program:
* allocating and freeing the scheduler
* passing it throughout the call stack in order to register the
  individual components (demuxers/decoders/filtergraphs/encoders/muxers)
  with the scheduler

The scheduler is not actually used as of this commit, so it should not
result in any change in behavior. That will change in future commits.

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